I think the difference between a rocket launcher and a recoilless rifle (or more correctly, recoilless gun) is that the projectile from a rocket launcher (a rocket) has a propellant that keeps the rocket moving after it has been fired, whereas a recoilless rifle has, much like a regular rifle, a 'bullet', which is propelled by a single combustion.
Rockets do, of course, not have any recoil. This is because they push against the air and not against the weapon. The principle of a recoilless rifle is that although the bullet 'pushes' against the rifle, there is an equal force in the opposite direction also 'pushing' against the rifle, so there is no net force on the rifle.